1. Field of the Invention
The present invention relates to an auto focus system, and in particular, to the auto focus system for controlling a focus of a picture-taking lens by a contrast method.
2. Description of the Related Art
An auto focus of a video camera and so on is generally depending on a contrast method. According to the contrast method, high frequency components of picture signals in a certain range (focus area) of the picture signals obtained from an image pickup device are integrated to be a focus evaluation value, and a focus adjustment is automatically performed so that the focus evaluation value becomes maximum (local maximum). It is thereby possible to obtain the best focus (focusing) for maximizing a degree of sharpness (contrast) of an image picked up by the image pickup device.
A so-called mountain climbing method is widely known as a method of setting a focus at a focusing position (a local maximum point of the focus evaluation value). According to this method, a direction in which the focus evaluation value increases is determined by comparing the focus evaluation values at two different points on moving the focus, and the focus is moved in that direction so that if the focus evaluation value turns to decrease from increase, the focus is returned to the position before the focus evaluation value decreased so as to set the focus at the local maximum point of the focus evaluation value.
In the case of the above-mentioned mountain climbing method, there is a drawback that an increasing direction of the focus evaluation value and focusing cannot be determined without actually moving the focus. Therefore, there is a proposed method whereby a focus state (front focus, rear focus or just focus) of a picture-taking lens is determinable without moving the focus by placing a plurality of image pickup devices at positions of different light path lengths (e.g., WO 02/099495 A1, WO 02/099496 A1, WO 02/099497 A1 and WO 02/099498 A1, which were, at the time the present invention was made, not published and not publicly known). According to this focus state determination method, it is possible to immediately know a current focus state from a magnitude relation among current focus evaluation values obtained from the image pickup devices so as to determine a moving direction of the focus and the just focusing without moving the focus. Therefore, the auto focus using this method has advantages such as being able to promptly set the focus at the focusing position.
In the case of moving a focus to a focusing position based on focus evaluation values obtained from a plurality of image pickup devices as mentioned above, a focus state is determined based on the focus evaluation values. Nevertheless, the focus evaluation values are not reflected on the control over movement of the focus until it is moved to the focusing position. For this reason, there is a problem that the movement of the focus until it is moved to the focusing position is unstable since it is not suited to characteristics of the focus evaluation values which are different depending on a state of an object and a picture-taking lens.